The present invention relates to the production of hydrocarbons. In particular, the present invention relates to primary and secondary catalysts for use in the Fischer-Tropsch process for synthesising hydrocarbons adapted for gasoline production and to supports for such primary catalysts.
The Fischer-Tropsch process is one of the major processes for producing synthetic hydrocarbons. The process is based on the use of carbon monoxide and hydrogen which can both be readily derived from gasification of coal or char or by steam reforming of natural gas. The process relies on catalytic synthesis and basically comprises passing a gaseous feed stream of carbon monoxide and hydrogen over a catalyst bed. The catalyst and process operating conditions are selected so that one or other of the following reactions predominates: EQU nCO+(2n+1)H.sub.2 .fwdarw.C.sub.n H.sub.2n+2 +nH.sub.2 O EQU nCO+2nH.sub.2 .fwdarw.C.sub.n H.sub.2n +nH.sub.2 O. EQU 2n CO+(n+1)H.sub.2 .fwdarw.C.sub.n H.sub.2n +nCO.sub.2 EQU 2n CO+nH.sub.2 .fwdarw.C.sub.2 H.sub.2n +nCO.sub.2
The Fischer-Tropsch process is subject to a number of limitations. One limitation is that the hydrocarbons produced by the process include a significant proportion of straight chain hydrocarbons of low octane number. Various processes have been proposed to improve the octane number of the hydrocarbons, and thus the suitability for use in gasoline production, by converting the straight chain hydrocarbons into branched hydrocarbons. However, the known processes tend to be expensive and are not adapted to form part of a continuous process with the Fischer-Tropsch process.
Another limitation is that it is difficult to control the process reaction to selectively form hydrocarbons having carbon numbers suitable for use in gasoline production. One reason for this particular limitation of the Fischer-Tropsch process is the selection of catalysts for use in the process. Iron is a major component of most existing commercial catalysts, and although it is active as a catalyst, the hydrocarbons produced cover a wide distribution of carbon numbers, many of which are not suitable for gasoline production. For example, depending on the process conditions, the hydrocarbons produced include significant proportions of either low carbon number gaseous products or high carbon number straight chain products which form as waxes. Both products are generally not suitable for use in gasoline production.
One object of the present invention is to provide an improved primary and an improved secondary catalyst, improved methods for the manufacture of the catalysts, improved supports for the primary catalyst and improved processes for using the catalysts which alleviate the disadvantages described in the preceding paragraphs.